We can look back further at the Hays code. That's just religion plain and simple. The feeling of, "we're sliding into a decadence which will lead to the downfall of our civilization" is a meme propagating this very sentiment. It's not a simple as just the government, but that does co-occur.
SPAM filters on SMTP ports were implemented long long before any government mandated it - at the ISP level often
Further, the development DKIM and SPF, were incidental to any government requirement
Preemptively: The fact that the early internet was heavily government and institutionally focused, doesn't a government mandate make